The economic impact on the United States alone will be in the trillions. Relocating all of Manhattan, Toronto, London, Singapore, and other coastal economic centers is non-trivial, but would hopefully be gradual enough to be viable. It would be fantastic if we can avoid this loss of wealth by making a meaningful reduction in the rate of global warming.
Other nations are already experiencing famine due to permanent desertification. This has already exacerbated political and military conditions; drought is a major factor in the Syrian and other war zones.
To compare this to the range of human history is not meaningful. "They had it worse" isn't helpful for now. What matters is our response to the conditions that we are expected to experience.
Around the 120,000 BCE time, there was a "great filter" event that caused the mass death of most of humanity, causing a massive reduction in human genetic diversity. Yes, we may have it "easier," but today's problems are still real and must be dealt with.
I think it's useful to frame the climate change process solely in terms of human impact, and specifically economic, political, and security terms to governments.
I wouldn't consider you a denier, and it's important to share your perspective. I hope you consider mine.